1. Field of the Invention
This invention relates to tape, disc, broadcasting and other sound information systems, and to solutions of the problems associated with noise reduction and the inherent limit of the dynamic range of such systems. The basic problem with respect to dynamic range of certain systems originates because of the difference in the dynamic range of the human ear which has a range of approximately 130 db. whereas tape, disc, and broadcast system usually have a dynamic range of less that 70 db. However, the problem isn't simply one of inadequate dynamic range of the sound reproduction system, it is further complicated by the fact that the human ear is a nonlinear device and therefore if the average level of a sound is reproduced different from that of the original sound, then the balance of gain between high and low frequencies is upset. As an example, if a recording is played back at a level lower than that of the original sounds, then the recording of the bass tones will sound deficient. Therefore, it will be appreciated that if a recorded sound is not reproduced at the same sound level as the original sound then some of form of compensation is necessary if the recorded reproduced sound is going to be asthetically pleasing to the listener. Of course, it is not essential that a sound be asthetically pleasing if the intelligence to be communicated is contained only in the words. However, if the reproduced sound is primarily for the purpose of the music then it is obvious that the asthetic qualities of the reproduced sound are of the utmost importance.
As is well known by those skilled in the art, each of the various sound reproduction systems have specific types of noise problems. Of course, simple frequency dependent fixed filters could be used to reduce the noise if the intelligence to be communicated was contained solely in the spoken word. Unfortunately, fixed filters will remove both signal and noise having a frequency in its operating range. Therefore, such filters cannot be used to eliminate noise in systems used for the reproduction of music as the reproduced signal would be audiably degraded.
More particularly, therefore, this invention relates to a new and unique detector suitable for controlling the variable gain amplifiers which are used in compressors, expandors, limiters and other noise-reduction systems. The invention also relates to new and unique designs of such compressors, expandors, compandors, limiters and noise-reduction systems which provide superior performance by making optimum use of the new and unique detector.
2. Discussion of the Prior Art
Dynamic range reduction and expansion systems as well as noise reduction systems fall into complimentary and noncomplimentary systems. The complimentary system minimizes distortion of the signal and is therefore the preferred method for high accuracy and sophisticated systems.
In a complimentary system, the signal is processed (compressed) before recording, and then processed again (expanded) during the playing of the recording, so as to substantially restore the signal to its original state.
The dynamic range of a substantial portion of the sounds generated by live entertainment, musical performances and other recordings, etc., which are ultimately heard by the human ear is frequently well in excess of that of any available recording medium. For purposes of this application the term dynamic range means the difference in decibels between the overload level of the system receiving the signal and the minimum signal level that can be received and processed by the system. Typically, human hearing is considered to have a dynamic range of approximately 130 decibels whereas the live performance of a large symphony orchestra will easily exceed 100 decibels. Under ideal circumstances, the best recording and broadcasting systems could have a dynamic range of close to 100 decibels for a selected pure tone. Unfortunately, the typical dynamic range of a system, is usually no better than 60 to 70 decibels for complex signals which contain many tones no matter whether the medium is tape, disc, etc. Consequently, it will be appreciated that an obvious solution might be to reduce the dynamic range of the signal being recorded by lowering the peak value of the signal below the upper limit of the recording medium and to increase the softer sounds so as to make them significantly louder than the noise. This is called compressing the dynamic range. A signal having a compressed dynamic range can then be recorded on the recording medium without excessive distortion. When the recorded and compressed signal is later "played back", the louder peaks are returned to their original level and the softer passages are again made softer. Thus the dynamic range of the recorded signal is expanded, and may then be amplified or otherwise made available to the human listener. Devices which can compress, expand, or both compress and expand are referred to as compressors, expandors, and compandors, respectively and represent a portion of the subject matter of this invention.
Signal compression by various means occurs universally in the broadcasting and recording industry. For example, during a broadcast or recording session the engineer may simply manually change the system input levels for any number of reasons. This type of dynamic range compression is commonly called "gain riding". In addition, circuits in the system may be employed to prevent the input volume from exceeding a selective level to avoid overmodulation. This type of dynamic range compression is called "limiting". It will be appreciated that the compressed program can be expanded back to its original dynamic range only if the expandor is exactly complimentary to the compressor. That is, the expandor must expand only those portions of the signal that were compressed and it must expand them precisely to the original level. Normally, in the case of "gain riding" or "limiting" the compression parameters are not known and therefore exactly complimentary expansion is not achieved. However, even in this type of operation, a general expander can be used to restore some of the dynamics, particularly if the parameters of the compressor can be adjusted so as to closely approximate the complimentary parameters of the compression system employed.
If the signal is compressed without later being complimentary expanded the sound reproduction system is what is known as a "non-complimentary" system. In other words, the reproduced signal would not be identical to the original.
In addition to its ability to compress and expand the dynamic range of a signal, the compandor, as will be discussed hereinafter, may also be used as a superior noise reduction system. However, it will be apreciated, that any noise reduction system employed in commercial broadcasting and recording industries must be standardized so that the recording may be played back through a complimentary system. Furthermore, any such compandor should produce listenable results when played back without complimentary expansion. This means, that expansion would not always be necessary for certain types of intelligence communication; however, if high fidelity was a major factor then the expansion could be performed and the performance restored substantially to its original dynamic range.
In a very general sense, all compandors (compressors, expandors) limiters and noise reduction systems consist of a variable gain amplifier responsive to a detected signal. Therefore, it will be appreciated that an essential element in all of these systems is a suitable detector for supplying the control signal which drives the variable gain amplifier. Therefore, a new and unique detector, suitable for use in superior compandors, limiters and noise reduction systems also represents the subject matter of this invention.
Various prior art detectors are discussed in more detail hereinafter. In particular, U.S. Pat. No. 2,585,890 issued to H. Wolfe; U.S. Pat. No. 3,187,268 issued to B. B. Bauer, and U.S. Pat. No. 3,260,957 issued to A. Kaiser; and a master's thesis submitted by the applicant of this invention disclose detectors which have a hold time (Th) in addition to attack time (Ta) and a decay time (Td). As used herein, the attack time Ta is defined as the time required for the detector output to come within 36.8% of its final value with a step input signal. For detectors not having a hold time, Th, the decay time Td, is defined as the time required for the detector output to drop to 63.2% of the final value and output for a sudden drop in input signal level. However, for systems having a hold time the sum of the decay time and hold time (Td+Th) is defined as the time required for the detector output to drop to 63.2% of its final change in output for a sudden drop in input signal level. The hold time, Th, is defined as the time the output remains constant after a drop in input level. If a detector has only an attack time and a decay time, the detector output will begin to change immediately after an input is removed. Then, if the input is applied again, detector output changes again. To reduce ripple caused by low frequency inputs, the decay time, Td, can be increased. Increasing Td when the detector is used in a compressor, limiter, expandor or noise reduction system minimizes the distortion and excessive gain changes. Unfortunately, a long Td is contrary to noise masking requirements. A long Ta will result in overshoot and is therefore incompatible with the requirements of a noise reduction systems. On the other hand if the attack time, Ta, is made short enough to prevent overshoot and Td is long, transient signals such as impulse noise or signals with high crest factors will cause a reduction of gain of the compressor or limiter (conversely and increase in gain of an expandor) for a substantial period of time after the change in the signal.
The addition of a hold time factor in a detector causes the output of the detector to remain constant for a time Th after a decrease in input signal level. If the input is increased back to its formal level within time Th, the detector output remains constant. But, if the input remains at the reduced level, the detection output decays with a time constant of Td after time Th has elapsed since the decrease of the input signal level. As a way of illustration, assume that Th is 50 milliseconds and Td is 100 milliseconds. If a full wave rectified sine wave of frequency greater than 10 Hz is applied to the input of the detector, the time between input signal peaks will be less than 50 milliseconds; hence, there will be no ripple in the detector output.
Subjectively, systems employing a short Ta and Td and a zero Th have been described as causing compressed sounds to sound distorted, full, muddy, or over-reverberant. On the other hand, if Td is made too long compressed music tends to sound slightly flat, empty or hollow.
Therefore, it can be seen that the detection system is very important since if affects Ta, tracking accuracy between expandor and compressor, and the listenability of compressed signals. The three types of detection presently available include peak detection as shown in prior art FIG. 1; true RMS detection as shown in FIG. 2; and true average detection, as shown in FIG. 3.
The output signal of a peak detector as is illustrated in FIG. 1 seeks the peak value of the input signal, and if the attack time, Ta, is short enough, the resulting output signal of the peak detector will increase to a level equal to the peak value of the input signal. A simple peak detector always has Ta less than the decay time, Td. As shown in FIG. 1, an input signal is received at input point 90 by a rectifying circuit 91 shown here as diode 92. Although a single diode 92 is illustrated it will be appreciated that a full wave rectifier would be even more preferable. The output of rectifying ciruit 91 is then applied to a non-linear smoothing filter 93A shown here as capacitive element 92 and resistive element 95. In the example illustrated in FIG. 1, which is for positive voltages, the smoothing filter is a low pass filter with a high cut-off frequency when the input signal is greater than the detector output Vo at point 96, and the filter has a lower cut-off frequency when the input signal is less than Vo.
The prior art RMS circuit detector of FIG. 2 includes squaring circuit 97 for squaring the input voltage received at point 90. Squaring circuit 97 is illustrated here as a multiplier circuit wherein both inputs to the multiplying circuit are the input voltage received at input point 90. Multiplication circuits of this type are commercially available. The output of squaring circuit 97 is then applied to linear low pass filter 93B (shown here as a resistive element 98 and a capacitive element 94 which is connected to ground) which provides a running time average output. The running time average output from low pass filter 93B is then applied to a division circuit 99 which uses the output at point 96 as the denominator of divider 99. Thus the square root of the running time average signal from filter 93 is obtained, and is by definition the RMS value of the input signal Vin received at input point 90. An RMS detector has a faster attack time, Ta, than its decay time, Td, and due to the square root operation, Ta and Td are dependent on the change in the input signal level.
The prior art average magnitude detector shown in FIG. 3 is very similar to the peak detector of FIG. 1 and differs substantially only in that smoothing filter 93B is linear. Therefore, the attack time, Ta, is equal to the decay time, Td. With many signals, the average magnitude is approximately equal to the RMS value. However, in practice the RMS value is usually slightly higher than the average magnitude value for an equivalent attack time, Ta. If the attack times, Ta, are equal, a peak detector provides the highest output value. Although some rather simplified detector circuits have been illustrated, it will be appreciated that commercial circuits which achieve these results are readily available and it is not necessary that they be built up from discrete components. For a more detailed discussion of such detector circuits, the reader is referred to T. H. Shingold's book entitled, "Non-Linear Circuits Handbook, Designing with Analog Functioning Modules and Ic's; 1976". The book is published by Analog Devices, Inc. of Norwood, Mass.
Because the ear determines loudness by the power of the signal, a RMS or an average peak detector will result in a more listenable compressed signal. In addition, RMS and average detection can sometimes result in better compandor tracking as RMS detection is insensitive to time delay distortion in the channel, while average detection is less susceptible to this distortion than peak detection, and RMS detection is more sensitive than the average detection to channel frequency response limitations. Unfortunately, good averaging requires that Ta be much longer than the period of the wave form. However, the most severe time delay distortion for transients occurs at high frequencies. But if a long Ta is employed, overshoot on compression of high level transient sounds will require the use of the limiter if clipping is to be avoided. Unfortunately, once a signal has been clipped or limited it cannot be restored by the expander as was discussed heretofore. An additional problem with RMS and average detection is that loud signals with high crest factors can easily overload the channel. Hence, it is apparent that best results will be obtained with RMS or average detection on all signals except those with high crest factors or high level transients where peak detection must occur.
The audibility of dropouts due to peak detection is strongly affected by Td and Th. The dropouts of any depth are inaudible if they last for no more than 10 milliseconds. This indicates that when peak detection is necessary, Td plus Th should be reduced to 10 milliseconds or less. This characteristic of a fast Ta and Td can be realized by taking the time derivative of the output of the average detector and adding this to the direct output of the average detector.
Of the various compandors systems that have been designed in an attempt to solve the problems of dynamic range, the so called "Dolby Systems" are perhaps the most successful. Dolby's approach set out in U.S. Pat. No. 3,631,365 which issued to Ray M. Dolby of London, England, compresses and expands various bands of the frequency spectrum individually.
Other U.S. patents issued to R. M. Dolby as an inventor or co-inventor which are related to the present invention include the following: U.S. Pat. No. 3,029,306 entitled Video Recording System and Method and Processing Amplified Network; U.S. Pat. No. 3,665,345 entitled Compressors and Expandors for Noise Reduction Systems; U.S. Pat. No. 3,737,678 entitled Limiters for Noise Reduction Systems; U.S. Pat. No. 3,760,102 entitled Level Setting in Noise Reduction Systems; U.S. Pat. No. 3,775,705 entitled Compressors and Expander Circuits having Control Network Responsive to Signal Level in Circuit; U.S. Pat. No. 3,818,244 entitled Limitors for Noise Reduction Systems; U.S. Pat. No. 3,828,280 entitled Compressors, Expanders and Noise Reduction Systems; U.S. Pat. No. 3,875,537 entitled Circuits for Modifying the Dynamic Range of an Input signal; and U.S. Pat. No. 3,865,416 entitled Signal, Compressors and Expandors. Although none of these Dolby inventions solve the problems in the unique and successful fashion as does the present invention.
A classical wide-band compandor system sometimes referred to as "The Burwen's Noise Eliminator" is described in U.S. Pat. No. 3,732,371 issued to R. S. Burwen.
Other Burwen patents related to the problem solved by the present invention include U.S. Pat. No. 3,678,416 entitled Dynamic Noise Filter having Means for Varying Cut-Off Point; and U.S. Pat. No. 3,753,159 entitled Variable Band Pass Dynamic Noise Filter. These last two patents being directed specifically to the noise filter problem whereas U.S. Pat. No. 3,732,371 discussed above is directed toward a compandor.
Still another classical wide-band compandor system is that described in U.S. Pat. No. 3,789,143 an issued to David E. Blackmer.
Other patents assigned to D. E. Blackmer include U.S. Pat. No. 3,681,618 entitled RMS Circuits with Bipolar Logarithmic Convertor and U.S. Pat. No. 3,714,462 entitled Multiplier Circuits. The Audimax.TM. system developed by CBS is widely employed in the broadcasting industry as a broadband compressor. Aspects of the Audimax.TM. system are described in U.S. Pat. No. 3,187,268 to B. B. Bauer; U.S. Pat. No. 3,260,957 to A. Kaiser; and U.S. Pat. No. 3,496,481 to E. Torick.
A compandor circuit also used by CBS and disclosed under U.S. Pat. No. 3,197,712 issued to Bauer entitled "Signal Compressor and Expander Apparatus" and U.S. Pat. No. 3,230,470 issued to Arther Kaiser and entitled "Compressor and Expander Apparatus".
Other patents assigned to CBS related to this field include U.S. Pat. No. 3,529,244 issued to Emil Torick and entitled "Method and Apparatus for Frequency Sensitive Amplitude Limiting"; and U.S. Pat. No. 3,582,964 issued to E. Torick et al, entitled "Automatic Loudness Controller".
Other compandor techniques are disclosed in U.S. Pat. Nos. 3,757,254 and 3,795,876 issued to Nobvaki Takahashi et al. Still another compandor system is described in U.S. Pat. No. 3,815,039 issued to Kiyoji Fujisawa et al and entitled "Automatic Noise Reduction System".
Still other patents related to specific aspects to this invention but not disclosing a compandor as such, include U.S. Pat. No. 3,206,556 to W. S. Bachman; U.S. Pat. No. 3,379,839 to J. H. Bennett; U.S. Pat. No. 3,238,457 to B. R. Boymel; U.S. Pat. No. 3,757,255 to J. P. Jarvis; U.S. Pat. No. 3,535,550 to G. S. Kang; and U.S. Pat. No. 2,585,890 to H. Wolfe.
Whereas all of the above listed and described patents attempt to solve the problems of dynamic range none of them succeeds in producing such asthetically pleasing recordings as do compandors using the new and unique detection of the present invention.
Experimental research effort related to this invention was the subject matter of a thesis by the applicant submitted to the Graduate Counsel of the University of Tennessee and entitled "Design Criteria of a Universal Compandor for the Elimination of Audible Noise in Tape, Disc and Broadcast Systems". Although the thesis which was published in December of 1974 is related to the present invention, it does not disclose, much less teach the solutions achieved by the new and unique detector circuitry of this invention.